The subject of the invention is a device for the separation of turbidities from liquids with permeation layers in the form of a depth filter having a specially designed filter bed, and a process for operating such a filter.
In the conventional mode of separating turbidities from liquids by means of depth filters as they are frequently used in the treatment of water, the filter bed generally consists of granular materials such as quartz, anthracite, pumice and the like. In this connection the following disadvantages are noted:
1. The predominant portion of the volume of the filter bed is taken up by the mass of the filter material which is inert with respect to the particle deposition process. PA1 2. The degree of particle separation in an elementary filter layer of the height of the filter grain diameter, is generally very small (below 1-2%). PA1 3. The conventional maximal turbidity deposit in the filter upon the attainment of which the depth filter needs to be regenerated either because of excessive increase in pressure drop or because of insufficient quality of the filtrate, is extremely small (in general notably smaller than 5% of the bed volume). PA1 4. Depending on the size of the filter material and the properties of the turbidities a reversal to cake filtration may occur, and this gives rise to an extremely strong increase in pressure drop across the filter so that the filter must be regenerated already after a very short period of operation. PA1 1. Added to the classic particle separation mechanisms in depth filters of granular materials, is, as a further mechanism in the case of the invention the convective transport due to permeation. PA1 2. Hydrodynamically conditioned retardation of the particle deposition on solid surfaces by liquid films which have to be displaced is greatly reduced. PA1 3. In the case of greatly structured membrane surfaces the microfibers can act for example as additional collectors and hence markedly enhance the separation efficiency. PA1 4. The sedimentation or flotation ability of turbidity particles can be better utilized compared with granular filter layers. PA1 5. By suitable selection and arrangement of the membrane layers the probability of particle adhesion can be substantially increased compared with granular filter layers. PA1 6. Adhesion of the particles is additionally improved by the permeation.
Especially the circumstances listed under 1 to 3 necessitate relatively large bed heights of depth filters, which in general lie between 1 and 3 m and require relatively low filtration velocities, that is approximately 5 m/h to 20 m/h.